Periodic or random vibrations or shocks can excite the resonant frequencies in a rotatable storage article which can be problematic due to the resultant formation of undesirable stresses, displacements, fatigue, and even sound radiation. Such undesirable vibrations or shocks are typically induced by external forces and can be experienced by a wide variety of articles and under a variety of conditions. For example, resonant vibrations can cause excessive vertical displacement of art optical disk's surface during operation which may lead to poor laser focus. Proper laser focus is a key to optimum write/read characteristics, signal quality, and tracking ability.
Various techniques have been used to reduce vibrational and shock effects (stresses, displacements, etc.) on storage articles. Three basic techniques to reduce vibration and shock effects include
1) adding stiffness or mass to the rotatable storage article so that the resonant frequencies of the rotatable storage article are not excited by a given excitation source, PA1 2) isolating the rotatable storage article from the excitation so the vibrational or shock energy does not excite the rotatable storage article's resonant frequencies, and PA1 3) damping the rotatable storage article so that given excitations do not result in excessive negative effects at the resonant frequencies of the rotatable storage article.
An isolation technique for limiting vibration transmission is described in U.S. Pat. No. 4,870,429 issued Sep. 26, 1989. A single-sided or double-sided optical disk structure is described which includes two sheets of substrate bonded to each other with a foam spacer interposed between the two substrates to restrict or isolate the vibrations caused by external forces. The spacer is made from an elastomeric foam material and is positioned between the two substrates to restrict the transmission of such forces (e.g. vibrations or shocks). The thickness of the spacer is stated to be preferably not less than 0.2 mm, more preferably not less than 0.4 mm, because when the thickness is too small the effect of the spacer to restrict or isolate forces is not exhibited sufficiently. Such a system adds to the overall size of the rotatable storage article and may be impractical where close positioning of the article to other structures is desired.
Two types of surface or external damping treatments which can be used to reduce shock or vibration impact on rotatable articles are: (1) free layer damping treatments; and (2) constrained layer damping treatments. Both of these damping treatments, Dan provide high levels of damping to a structure, i.e., dissipation of undesirable vibrations, without sacrificing the stiffness of the structure. The use of viscoelastic materials as exterior surface damping treatments is described in EP 0507515 published Oct. 7, 1992. Examples of additional surface or external damping techniques are described, for example, in U.S. Pat. Nos. 2,819,032 (issued Jan. 7, 1953); 3,071,217 (issued Jan. 1, 1963); 3,078,969 (issued Feb. 26, 1963); 3,159,249 (issued Dec. 1, 1964); and 3,160,549 (issued Dec. 8, 1964). All of the aforementioned damping techniques can add complexity and expense to the design of the rotatable storage article, limit the amount of exterior article surface available for data storage, and can increase the overall size of the article.
Free layer damping; treatment is also referred to as "unconstrained layer" or "extensional damping" treatment. In this technique, damping occurs by applying a layer of viscoelastic damping material to one or more exterior surfaces of the article to be damped. The material can be applied to one or more exterior surfaces of the article to be damped. The mechanism by which this treatment method dissipates undesirable energy, e.g., resonant vibrations, involves deformation. That is, when the article is subjected to cyclic loading, for example, the damping material is subjected to tension-compression deformation and dissipates the energy through an extensional strain mechanism.
Constrained layer damping treatment is also referred to as "shear damping" treatment. For a given weight, this type of damping treatment is generally more efficient than the free layer damping treatment. In this technique, damping occurs by applying a damper consisting of one or more layers of viscoelastic damping material and one or more layers of a higher tensile modulus material to one or more exterior surfaces of the article to be damped. That is, this damping technique is similar to the free layer damping treatment wherein a viscoelastic material is applied to one or more exterior surfaces of a structure, the difference being that the viscoelastic material is additionally constrained by a layer having a higher modulus than the viscoelastic material, e.g., a metal layer, in the constrained layer treatment. Energy dissipates from the viscoelastic damping material via a shear strain mechanism. The shear strain results from constraints by the higher modulus constraining layer and the base structure.
Although these exterior surface damping techniques are used, the degree of damping is oftentimes limited by thickness or spacing requirements as well as application difficulties. Furthermore, the exterior damper must be applied to potential data storage surface areas, limiting information storage capability. In addition, external dampers can interfere with information retrieval from the storage article. Another disadvantage is that the external damper may be subject to degradation by the environmental conditions in which it is used. As way of example, if a rotatable storage article is desired to be a component in a size limited application, such as hard disk drives for portable computer systems, computers, or calculators, the ability to adequately damp the rotatable storage article by means of an "add-on" exterior surface damper may not be possible due to overall thickness requirements to meet a "form factor" requirement or the necessity of using the exterior surface for data/information storage. Thus, an alternative approach is needed to damp vibrational or shock energy without adversely affecting the overall size or thickness or available surface area of the rotatable storage article.